In treatment of cancer, for example, a lesion of a patient may be irradiated with radiation. In such radiotherapy, a particle radiotherapy apparatus which uses a particle beam has been developed (see Nonpatent Document 1, for example).
Such a particle radiotherapy apparatus will be described. As shown in FIG. 12, a conventional particle radiotherapy apparatus 51 includes a top board 52 for supporting a patient M, a particle beam source 53 for emitting a particle beam, and a first detector ring 54 and a second detector ring 55 for detecting annihilation gamma ray pairs generating from inside the patient M. The particle beam source 53 is disposed in a position between the two detector rings 54 and 55. And the particle beam source 53 can move around the patient M, about the body axis of the patient M. That is, a gap provided between the two detector rings 54 and 55 serves as a passage of the particle beam.
The construction of the two detector rings 54 and 55 will be described. The first detector ring 54 is constructed of blockish radiation detectors 61 arranged in a ring form. As shown in FIG. 13, this radiation detector 61 has a scintillator 62 for converting the radiation into fluorescence, and a photomultiplier tube (hereinafter called photodetector) 63 for detecting the fluorescence. The scintillator 62 has rectangular parallelepiped-shaped scintillator crystals C arranged in three dimensions. The photodetector 63 can determine which scintillator crystals C have emitted the fluorescence. That is, the radiation detector 61 can specify where on the scintillator 62 the radiation is incident. Of the surfaces of the scintillator 62, the surface remotest from the photodetector 63 will be called the plane of incidence 62a for expediency.
Next, a sectional view of the conventional particle radiotherapy apparatus 51 is shown. As shown in FIG. 14, the two detector rings 54 and 55 in the conventional particle radiotherapy apparatus 51 have the radiation detectors 61 arranged simply. That is, the scintillators 62 are directed inward of the two detector rings 54 and 55. Specifically, the scintillators 62 are directed to the same positions in the body axis direction A of the patient M.
When carrying out radiotherapy with the particle radiotherapy apparatus 51, a particle beam is emitted from the particle beam source 53 to the patient M placed on the top board 52. The particle beam source 53 moves around the body axis of the patient M while emitting the particle beam, and continues emitting the particle beam to the patient M while changing irradiation angle. The particle beam loses energy in the body of the patient M. At this time, the nucleus located at the point where the particle beam loses energy is converted into a nuclide which causes β+ decay. This nucleus causes β+ decay and emits a positron. The resulting positron encounters and annihilates with an electron present in the vicinity. At this time, a pair of annihilation gamma rays are produced, which move in 180° opposite directions. This annihilation gamma ray pair penetrate the patient M, and are detected by the two detector rings 54 and 55. The conventional particle radiotherapy apparatus 51 determines the location where this annihilation gamma ray pair have been produced, thereby to presume a point where the particle beam lost energy. Cells are destroyed adjacent the point where the particle beam lost energy. In this way, it can be found out whether the particle beam accurately aims at the lesion of the patient M. The annihilation gamma ray pairs are an example of radiation resulting from the particle beam.
In order to attain the object of determining the location where the annihilation gamma ray pair have been produced, both of the annihilation gamma ray pair must be detected. This is because the point of occurrence of the annihilation gamma ray pair is determined by obtaining a line (Line of Response: hereinafter referred to as LOR as appropriate) extending between two points where the annihilation gamma ray pair are detected.    [Nonpatent Document 1] “IEEE Nuclear Science Symposium Conference Record) (U.S.A), November 2007, No. 5, p 3688-3690